Liquid crystal display devices are used in various fields as OA devices such as televisions and personal computers, mobile terminal devices, and image display devices such as digital signage. Together with an increase in functionality of devices such as televisions, OA devices and mobile terminal devices described above, on which liquid crystal display devices are mounted, improvement in display performance and reduction in power consumption of liquid crystal display devices are required.
A liquid crystal display device has a display panel including a liquid crystal layer sandwiched between two alignment films and a backlight which supplies light to the display panel. A CCFL (Cold Cathode Fluorescent Lamp) and a light emitting diode (LED: Light Emitting Diode) are examples of a light source of a backlight used in a liquid crystal display device.
In a liquid crystal display device, one pixel is generally formed from three subpixels which display red, green and blue (RGB) which are three primary colors of light. In recent years, in order to improve image resolution, luminosity and reduce power consumption, an image display device has been proposed in which a pixel including four subpixels displaying red, green, blue and white (RGBW) with a white (W) subpixel generally added to red, green and blue (RGB) subpixels is applied to a display panel. In such an image display device, an RGB image signal input from the outside is converted to an RGBW image signal suitable for the image display device. Since it is possible to improve luminosity efficiency by using a white (W) subpixel, it is possible to reduce the amount of light emitted by the backlight.
In addition, a method is known in which power consumption of the backlight is reduced by elongating an image signal and reducing the amount of light emitted from the backlight by the amount corresponding to the elongation of the image signal.